A naturally aspirated internal combustion engine may include exhaust gas recirculation (EGR) to reduce engine pumping work and NOx emissions. Exhaust gases may be returned to an engine's intake manifold when intake manifold pressure is lower than exhaust manifold pressure. The lower intake manifold pressure provides a motive force to draw exhaust gas from the exhaust manifold to the intake manifold. A naturally aspirated internal combustion engine may be configured with intake valves that close late in a compression stroke of a cylinder such that a portion of charge in the cylinder (e.g., air and fuel) may be pushed back into the engine's intake manifold. By pushing a portion of charge back into the engine intake manifold, the engine may be operated at a higher intake manifold pressure, thereby reducing engine pumping work. However, late intake valve closing may make it more difficult to flow a desired amount of EGR to engine cylinders since a smaller pressure differential may exist between the engine intake manifold and the engine exhaust manifold. Further, late intake valve closing has two effects on engine knock, one positive effect, one negative effect. Late intake valve closing reduces the engine's effective compression ratio which reduces compression heating, but this benefit may be offset by heating of the charge pushed back into the intake, the charge heated via cylinder walls, the cylinder head, and heat from the piston. Therefore, it would be desirable to provide an engine that has the advantages of late intake valve timing and EGR without the disadvantage of pushback charge warming.
The inventor herein has recognized potential issues with such systems and has developed a method that may lower an engine's propensity to knock while reducing engine NOx emissions and engine pumping work. In particular, the inventor has provided an engine method comprising: ejecting a second portion of exhaust gas from combustion in the cylinder during the cycle to a scavenge manifold at a first time, the cylinder included in a naturally aspirated engine; and ejecting fresh air from the cylinder to the scavenge manifold during the cycle of the cylinder at a second time, the second time different from the first time.
By ejecting exhaust gas and fresh air from a cylinder to a scavenge manifold, it may be possible to push fresh air and exhaust gas to an engine intake manifold so that cooled EGR may be provided to engine cylinders. The cooled EGR may reduce NOx, and pushing fresh air back into the intake manifold may allow the engine to operate at higher intake manifold pressures to reduce engine pumping work. Further, the engine's propensity to knock may be reduced since the EGR is cooled and any air that was warmed during the intake can be recirculated and re-cooled.
The present description may provide several advantages. For example, the approach may reduce engine pumping work, thereby reducing engine fuel consumption. In addition, the approach may reduce an engine's propensity to knock so that the possibility of engine degradation may be reduced. Further, the approach may provide EGR to engine cylinders during wide open throttle conditions to lower production of NOx within the engine.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.